Fluoride is bad because it is unnecessary. Sodium fluoride has long been thrown out and is no longer used in water fluoridation treatments. The new thing is now fluorosilicates like hexafluorosilicic acid, which have no studies regarding efficacy or health effects, unlike sodium fluoride - despite that research being highly suspect.

The truth is hexafluorosilicic acid and other fluorosilicates are waste products and are added to water supplies in order to dampen the costs of properly disposing of them.